1. Field of the Invention
This invention relates to a means for diminishing as rapidly as possible the segregation and porosity of the center of the cast piece.
2. Description of the Related Art
One important issue in continuous casting is how to alleviate the segregation and center porosity occurring in the center of the cast piece. Good results are being obtained by use of electromagnetic stirring and low temperature casting to prevent segregation or additives to promote heterogeneous nucleation, and segregation dispersion technology is implemented by bulk forming of equiaxed crystal. Further, high level purifying processes are being introduced to reduce concentrations of impurities (phosphorus and sulfur etc.) in molten steel. Anti-bulge technology is being implemented during the cast piece drawing out process.
However it is clear that countermeasures are inadequate, for the process in which segregation is brought about by the molten metal flow accompanying solidification shrinkage during final solidification or the process in which center porosity is formed as a direct result of said solidification shrinkage.
Whereupon in recent years continuous casting technology has been proposed using several pair of reduction rollers in the final stage of the cast piece draw out process and applying reduction at a low reduction rate to cast pieces in the final solidification stage. By applying this kind of reduction, the said molten flow can be inhibited to help prevent segregation, and solidification shrinkage can be corrected, to prevent formation of center porosity, ultimately allowing manufacture of a continuous casting product having no defects.
Technology contributing towards this reduction method is known in such public literature as Japanese Laid-open No.s 59-16862, 3-6855, 3-8863, 3-8864, 4-20696, 4-22664 and 5-30548. This public literature on the position (namely the interval from starting the reduction until finish of the reduction) for reduction all indicates a common concept. However the above methods proposing for instance technology to control the reduction rate (within 1.5%) or quantity of reduction rate (0.5 mm per minute to 2.5 mm per minute) or reduction at 0.6 .epsilon. mm per minute to 1.1 .epsilon. mm per minute (.epsilon. is a reciprocal number of 1/4th of the flatness ratio of cast pieces) still do not provide any decisive condition of reduction.
There is however specific equipment technology for performing the reduction as provided for instance in methods in Japanese Patent Laid-open No.s 50-55529 and 54-38978 for a roll (generally termed a flat roll) of the same width as the cast piece or roll barrel longer than the cast piece width, as for instance the roll as revealed in Japanese Patent Laid-open No. 2-56982 wherein the diameter of center part of roll is made slightly larger than that of the other part (referred to in this invention as a stubby roll).
The above existing reduction technology has the haphazard approach of trying to define reduction conditions such as the extent of reduction required, leaving the basic problem unsettled. Further, till now progressive changes occurring in the center solid fraction during the reduction process has not been considered and nothing has been revealed regarding how to alter the extent of the reduction in response to changes in the center solid fraction. For instance, in dealing with internal cracks prone to occur in high carbon steel or in particular with proper reduction conditions for bloom continuous casting, the studies done up until now can only called totally insufficient.